Bar code production identification systems have exhibited extensive utility in a broad range of industries. Generally, the codes are comprised of sequences of dark lines or bars located over or against a white or highly contrasting background. These bars are of selectively varying thickness and spacing so as to be machine readable and, for the most part, are provided as labels on goods or devices. To achieve a reliability for readability, the bar codes employ the basic principle of establishing a very high contrast between a white background and a black bar or vice versa. In this regard, the specifications for the production of these codes are very rigid, an adequate ratio of reflectance and absorbance in the produced code being required.
The retail trade utilizes the coded systems extensively to facilitate partial automation of the retail store check-out process. As items carrying the bar coded labels are presented to the store clerk or checker, they are passed over an optical scanner which employs laser optics for "reading" the code and transmitting the thus read information to a computer for any of a wide variety of purposes related, e.g. to price, inventory, taxing, special programs, or the like. For effective and reliable operation, the goods carrying the coded labels are manipulated by the operator or checker within the defined optical scanning region of the code detection system until a valid "read" is achieved. Generally, a short audible pulse indicates the reception of a valid read and the rate thus required for this reading procedure is of a slow enough level to permit a real time computer analysis of the code as it is being scrutinized bar by bar.
Industry requirements for product identification now have expanded. For example, it has been found to be desirable to provide bar codes which are formed as part of the product manufacturing process itself. Where bar codes are to be so used, the bar-background contrast ratios necessarily must be greatly weakened. For instance, one industrial field, the tire industry, has identified a need for applying a bar code identification to tires at the time of their molding. This bar code would be a series of bar protrusions extending from the outer surface of the tire itself. Necessarily, there will be no strong visual contrast between the protruded bar components and the background, inasmuch as they are formed from the same black pigmental material and the code structuring which is provided might be considered as "black-on-black". Thus, for any identification system of this type to be practical, a technique for reading this molded bar code of minimal reflectance ratio must be devised.
Where such reading of "black-on-black" codes is achieved, a next development requirement is imposed upon the investigator. Because industrial production procedures generally involve the regulated movement of products along a production route or line, bar code reading must occur at select points along that route. For the reading procedure to be practical, no operator intervention should be required to carry out the act of reading. For example, no operator should be present to manipulate the tire or other product before the scanning opticals to search for a valid read signal as is common in connection with retail outlet utilizations of the systems. Thus, a principal difficulty in evolving a practical code reading system is a requirement for identifying with accuracy where the code is located on the product in the first instance. For the case of tire manufacture, the code might exist in any region over a two inch or more diametric region on the tire simply because tires are produced in differing sizes. Yet, to achieve a valid read, the optical scanning system must accurately confront or cross each bar within the entire code. Attempts to carry out such reading with conventional optical scanning devices have been observed to result in unsatisfactory performance.